This invention relates to an organic thin film electroluminescent device.
In keeping with the progress towards a more intelligent society, various display devices have been developed. Of these, electroluminescent devices, referred to hereinafter as EL devices, are attracting attention as ideal fully solid planar light emitting devices.
Heretofore, as an EL device, a device having a light emitting layer formed of ZnS:Mn, which is an inorganic light emitting agent, sandwiched between two electrodes, has been used extensively. Such an EL device, however, has a defect that, since its operation is based on a so-called impact excitation type light emitting mechanism, it needs a driving voltage in excess of 100 V (a.c. voltage) and is low in light emitting efficiency and luminance. On the other hand, since the number of available inorganic light emitting materials is limited, the number of the colors that can be displayed is limited raising difficulties in achieving a full-color display.
For this reason, there has recently been proposed a variety of injection type organic thin film EL devices operating under a light emitting mechanism different from that of the conventional EL devices. For example, there are proposed by C. W. Tang et al. an EL device in which, to produce for light emission of high luminance comparable to that of a light emitting diode under low d.c. voltage of the order of several tens volts, an arylamine compound is evaporated between a light emitting layer and a hole injection electrode, to form a layer that functions as an organic hole transport layer, and in which an electron injection electrode is formed of a metal, such as magnesium, having a low work function; an EL device in which a light emitting electron transport layer and a hole injection layer are sandwiched between two electrodes; and an EL device in which a dopant such as cumarine is doped in a light emitting electron transport layer and in which phosphorescent light emitted from the dopant is used as an emission light source (Appl. Phys. Lett. 51 (12), 913 (1987)). There is also proposed by Adachi et al. an EL device in which a hole transport layer, a light emitting layer and an electron transport layer are sandwiched between two electrodes (Jpn. Journal of Applied Physics, 27, No. 2 L 269 (1988)).
However, these EL devices have a disadvantage that, since a monomer is employed as a component constituting the electron transport layer and the hole transport layer, these layers need to be formed by evaporation or a like method, so that it is difficult to render the thicknesses of the layers constant and hence non-uniformities are produced in the luminance or colors of the devices. In addition, since pinholes or similar defects are likely to be produced so that shorting may occur in the device, render the device non-luminescent to adversely affecting neighboring devices. On the other hand, since the various layers are formed by evaporation, it is difficult to increase the area of the layers.